A mixed power transmission line is a transmission line that consists of overhead and underground line sections. At the very least, a mixed power transmission line has one overhead line section and one underground line section. Such a transmission line is preferred over a homogenous transmission line (i.e. a line that has only an overhead section(s), or a line which only consists of an underground cable(s)) for a variety of reasons including, but not limited to, safety considerations, ease of accessibility of substations in thickly populated locations, ease of grid connection between countries with varying topology, and need for subsea electrification. An example where a mixed power transmission line is preferred instead of a homogenous transmission line, is when the transmission line is expected to traverse across natural vegetation, rivers etc.
As in case of a homogenous transmission line, faults may occur in a mixed power transmission line. Overhead sections of the mixed power transmission line are more prone to faults as they are exposed to disturbances such as lightning strikes, wind-borne debris and the like. In case of homogenous transmission lines, an auto-reclose functionality is utilized, albeit depending upon the type (underground/overhead) of the homogenous transmission line. A switching device such as a circuit breaker, or an auto-recloser may be utilized for the purpose of disconnecting a faulty line from the rest of the power transmission network, thereby mitigating the risk associated with propagation of a fault(s). Typically, the faults that occur in overhead transmission lines are transient (temporary) in nature, and therefore, such transient faults can be dealt with using an auto-recloser. However, in case of underground transmission lines, which are more prone to permanent faults, auto-recloser functionality cannot be used.
Therefore, in case of power transmission networks comprising mixed transmission lines, auto-reclosers need to be controlled in a selective manner. In case of occurrence of fault in an underground section, the auto-recloser operation should be disabled, given the permanent nature of the fault. Therefore, there was felt a need for a system and method that is effective and efficient in terms of identifying an occurrence of a fault in a mixed power transmission line, and providing for appropriate and efficient protection.